Incorporate `iter_nodes` into `graph::DirectedGraph`
This helper method iterates over all node IDs in the dense range `0..num_nodes`.
In practice, we have a lot of graph-algorithm code that already assumes that nodes are densely numbered, by using `num_nodes` to allocate per-node indexed data structures. So I don't think this is actually a substantial change to the de-facto semantics of `graph::DirectedGraph`.
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Resolves a FIXME from #135481.
f29979aebe